One thing I enjoyed about BW was the whole Elders (or Sages? I can't remember) side-quest that happened after beating the main story. I also liked the sheer quantity of areas that you could visit afterwards. It was significantly more than any other generation before it and I enjoyed the feeling of "Only half the game is over."

Obviously the National Dex will become a thing (assuming they don't just flat-out make no such thing this generation), some sort of Frontier (if not in this first set of games, in the third / fourth one), and possibly some sort of "fight gym leaders" again system. I can't imagine there's much left in terms of end-game other than these.

Perhaps they can make legendary captures a bit more "quest" oriented instead of "go to this cave and find a random legendary and catch it I guess." I just want the game to be a bit more cinematic.